The present invention relates to the field of light emitting diode based lighting and more particularly to a means of using a reduced number of color and thermal sensors in a scanning backlight.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display. In a large LCD matrix display typically the LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current.
In order supply a white backlight for the matrix display one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique one or more individual strings of colored LEDs are placed in proximity so that in combination their light is seen a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the matrix display, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused so as to avoid hot spots by a diffuser. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design in manufacturing is centered on the need for a correct white point.
Each of the colored LED strings is typically intensity controlled by both amplitude modulation (AM) and pulse width modulation (PWM) to achieve an overall fixed perceived luminance. AM is typically used to set the white point produced by the disparate colored LED strings by setting the constant current flow through the LED string to a value achieved as part of a white point calibration process and PWM is typically used to variably control the overall luminance, or brightness, of the monitor without affecting the white point balance. Thus the current, when pulsed on, is held constant to maintain the white point among the disparate colored LED strings, and the PWM duty cycle is controlled to dim or brighten the backlight by adjusting the average current. The PWM duty cycle of each color is further modified to maintain the white point, preferably responsive to a color sensor. The color sensor is arranged to receive the white light, and thus a color control feedback loop may be maintained. It is to be noted that different colored LEDs age, or reduce their luminance as a function of current, at different rates and thus the PWM duty cycle of each color must be modified over time to maintain the white point.
One known problem of LCD matrix displays is motion blur. One cause of motion blur is that the response time of the LCD is finite. Thus, there is a delay from the time of writing to the LCD pixel until the image changes. Furthermore, since each pixel is written once per scan, and then is held until the next scan, smooth motion is not possible. The eye notices the image being in the wrong place until the next sample, and interprets this as blur or smear.
This problem is resolved by a scanning backlight, in which the matrix display is divided horizontally into a plurality of regions, and the backlight for each region is illuminated for a short period of time in synchronization with the writing of the image. Ideally, the backlighting for the region is illuminated just after the pixel response time, and the illumination is held for a predetermined illumination frame time.
In a preferred embodiment, each region is provided with optical partitions to prevent spillover of illumination from one region to an adjacent region, and the regions are stacked horizontally. Thus, in an embodiment in which colored LED strings are supplied, the color for each region must be controlled. U.S. Pat. No. 6,870,525 issued Mar. 22, 2005 to Kawabata et al., the entire contents of which is incorporated herein by reference, teaches a separate light sensitive element for each region. Such a method is quite costly in the case of LED backlighting, as a separate color sensor will then be required for each region.
World Intellectual Property Organization International Publication S/N WO 2005/111976 published Nov. 24, 2005 to Fisekovic et al, the entire contents of which is incorporated herein by reference, is addressed to a scanning backlight for a matrix display. A sensing signal responsive to a plurality of lighting sources is supplied, the sensing signal being sampled at different times in coordination with the scanning period. Thus, a single sensor is responsive to a plurality of lighting sources. Unfortunately, as the effectiveness of optical partitions improve, thereby improving the operation of the scanning backlight and the matrix display as a whole, such a single sensor will not receive sufficient light from adjacent regions to be efficient.
U.S. Pat. No. 6,411,046 to Muthu issued Jun. 25, 2002, the entire contents of which is incorporated herein by reference, is addressed to a method of controlling the light output and color of LEDs in a luminaire by measuring color coordinates for each LED light source at different temperatures, storing the expressions of the color coordinates as a function of the temperatures, deriving equations for the color coordinates as a function of temperature, calculating the color coordinates and lumen output fractions on-line, and controlling the light output and color of the LEDs based upon the calculated color coordinates and lumen output fractions.
What is needed, and not provided by the prior art, is a means for controlling the color of a plurality of LED backlighting strings separated by optical partitions without requiring a separate color sensor and thermal sensor for each region.